Generation of pulsed plasma with pulses and off-periods of relatively short duration presents a unique set of challenges. There are several limitations of the presently known plasma generating devices that make their use for generating pulsed plasma impracticable.
Generally, a plasma generating device comprises a cathode and an anode. A plasma generating gas, which is typically a noble gas, flows in a channel extending longitudinally between the cathode and through the anode. As the plasma generating gas traverses the plasma channel it is heated and converted to plasma by an electric arc established between the cathode and the anode. Portions of the plasma channel may be formed by one or more intermediate electrodes.
Generation of plasma occurs in three phases. The first phase, called a spark discharge, occurs when an electric spark is established between the cathode and the anode. The second phase, called a glow discharge, occurs when positively charged ions, formed as a result of the motion of negatively charged electrons in the electric spark, bombard the cathode. The third phase, called an arc discharge, occurs after a portion of the cathode is sufficiently heated by the ion bombardment that it begins to emit a sufficient number of electrons to sustain the current between the cathode and the anode for heating the plasma generating gas. The electric arc heats the plasma generating gas, which forms plasma. Each time high temperature plasma is generated, the plasma generating gas has to go through all three phases.
In the prior art devices, at startup, the current passing between the cathode and the anode is simply raised to the desired operational level. This rapid increase in the current, however, cannot be sustained during the spark discharge and glow discharge phases. Only once the arc discharge phase is reached and the cathode begins thermionically emitting electrons with a rate sufficient to support such a current, the applied operational level current begins to flow between the cathode and the anode. Attempting to pass a high, operational level, current through the cathode before it begins to thermionically emit electrons with sufficiently high rate to sustain such current exerts stress on the cathode, which ultimately causes its destruction after a relatively low number of startups.
Generation of pulsed plasma requires frequent startups of the plasma generating device in a rapid succession. For example, in skin treatment, a single session of treatment with pulsed plasma may require thousands of pulses and consequently thousands of startups. The prior art methods of starting up plasma-generating devices are unsuitable for pulsed plasma generation because the cathode may be damaged during the session.
Presently, two types of devices may be used for generation of pulses of ionized gas. The device disclosed in U.S. Pat. No. 6,629,974 is an example of the first type. In devices of this type, a corona discharge is generated by passing plasma generating gas, such as nitrogen, through an alternating electric field. The alternating electric field creates a rapid motion of the free electrons in the gas. The rapidly moving electrons strike out other electrons from the gas atoms, forming what is known as an electron avalanche, which in turn creates a corona discharge. By applying the electric field in pulses, pulsed corona discharge is generated. Among the advantages of this method for generating pulsed corona discharge is (1) the absence of impurities in the flow and (2) short start times that enable generation of a truly pulsed flow. For the purposes of this disclosure, a truly pulsed flow refers to a flow that completely ceases during the off period of the pulse.
A drawback of devices and methods of the first type is that the generated corona discharge has a fixed maximum temperature of approximately 2000° C. The corona discharge formed in the device never becomes high temperature plasma because it is not heated by an electric arc. Therefore, devices that generate pulsed corona discharge cannot be used for some applications that require a temperature above 2000° C. Accordingly, applications of devices of the first type are limited by the nature of the electrical discharge process, that is capable of producing a corona discharge, but not high temperature plasma.
Devices of the second type generate plasma by heating the flow of plasma generating gas passing through a plasma channel by an electric arc that is established between a cathode and an anode that forms the plasma channel. An example of a device of the second type is disclosed in U.S. Pat. No. 6,475,215. According to the disclosure of U.S. Pat. No. 6,475,215, as the plasma generating gas, preferably argon, traverses the plasma channel, a pulsed DC voltage is applied between the anode and the cathode. A predetermined constant bias voltage may or may not be added to the pulsed DC voltage. During a voltage pulse, the number of free electrons in the plasma generating gas increases, resulting in a decrease in the resistance of the plasma and an exponential increase of the electric current flowing through the plasma. During the off period, the number of free electrons in the plasma generating gas decreases, resulting in an increase in resistance of the plasma and an exponential decrease in the current flowing through the plasma. Although the current is relatively low during the off period, it never completely ceases. This low current, referred to as the standby current, is undesirable because a truly pulsed plasma flow is not generated. During the off period a continuous low-power plasma flow is maintained. In essence, the device does not generate pulsed plasma, but rather a continuous plasma flow with power spikes, called pulses, thus simulating pulsed plasma. Because the off-period is substantially longer than a pulse, the device outputs a significant amount of energy during the off period and, therefore, it cannot be utilized effectively for applications that require a truly pulsed plasma flow. For example, if the device is used for skin treatment, it may have to be removed from the skin surface after each pulse, so that the skin is not exposed to the low power plasma during the off period. This impairs the usability and safety of the device.
Dropping the current flow through the plasma to zero between pulses and restarting the device for each pulse of plasma is not practicable when using the device disclosed in U.S. Pat. No. 6,475,215. Restarting the device for each pulse would result in the rapid destruction of the cathode, as a result of passing a high current through the cathode without ensuring that it emits enough electrons for the plasma flow to support this current. Attempting to pass a high current through the cathode before it begins to emit electrons with sufficiently high rate to sustain such current exerts stress on the cathode, which ultimately causes its destruction. Alternatively, it is possible to increase slowly both the voltage between the cathode and the anode and the current passing through the plasma. This alternative is not practical either because the startup of the device for each pulse would be impermissibly long.
The inability of the device disclosed in U.S. Pat. No. 6,475,215, and other devices of this type presently known in the art, to generate a truly pulsed plasma flow is due to the structure of the device. When devices of this type startup there is some erosion of electrodes due to sputtering. This erosion results in separated electrode materials, such as metal particles, flowing in the plasma. When a continuous plasma flow is used, the startup impurities are a relatively minor drawback, because the startup, and the impurities associated with it, occur only once per treatment. It is therefore possible to wait a few seconds after the startup for the electrode particles to exit the device before beginning the actual treatment. However, waiting for impurities to exit the device when using a pulsed plasma flow is impractical because particles separate from electrodes for each pulse.
When the plasma flow has been previously created it takes just a few microseconds to increase or decrease the current in the plasma flow. Additionally, because there are no startups during treatment, impurities do not enter the plasma flow, and there is no stress on the cathode. However, sustaining even a low electrical current through the plasma continuously renders the device suboptimal for some applications that require a truly pulsed plasma flow, as discussed above.
Difficulties in generating a truly pulsed plasma flow by the means of heating the plasma generating gas with an electric arc are primarily due to the nature of the processes occurring on the cathode and the anode. In general, and for medical applications especially, it is critical to ensure operation free from the erosion of the anode and the cathode when the current rapidly increases. During the rapid current increase the temperature of the cathode may be low and not easily controlled during subsequent repetitions of the pulse. During the generation of an electric arc between the cathode and the anode, the area of attachment of the arc to the cathode strongly depends on the initial temperature of the cathode. When the cathode is cold, the area of attachment is relatively small. After several pulses the temperature of the cathode increases, so that during a rapid current increase the area of attachment expands over the entire surface area of the cathode and even over a cathode holder. Under these circumstances, the cathode fall begins to fluctuate and the cathode erosion begins. Furthermore, if the area of attachment of the electric arc reaches the cathode holder it begins to melt thus introducing undesirable impurities into the plasma flow. For the proper cathode functionality, it is necessary to control the exact location and the size of the area of attachment of the electric arc to the cathode surface during rapid current increases in each pulse of plasma.
An electric arc tends to attach to surface imperfections (also called irregularities) on the cathode. In the prior art, such surface imperfections were created by altering the shape of a cylindrical cathode. A typical surface imperfection used in the prior art is cathode tapering. Cathode tapering creates a tip to which the arc tends to attach. Another way to create an imperfection is by cutting a cylindrical cathode at an angle. This too creates an imperfection to which the arc tends to attach. Although these methods control the location of the electric arc attachment between continuous plasma flow sessions, they are not sufficient for controlling the size of that area for the pulsed plasma operation due to the gradual expansion of the area of the arc attachment, as described above.
Independently from these attempts of controlling the location and size of the area of the arc attachment, some prior art devices used multiple cathodes for various purposes. For example, in U.S. Pat. No. 1,661,579 multiple cathodes were used in a plasma-based light bulb for generating a spark between them. In U.S. Pat. No. 2,615,137 a plurality of cathodes are divided in three groups. Three-phase power is distributed between the cathodes so that one group is used during a phase for providing a pseudo-continuous mode of operation. In U.S. Pat. No. 3,566,185 a pair of cathodes is used for sputtering of metallic traces from the cathodes by using particles isolated between the cathodes by a magnetic field. In U.S. Pat. No. 4,785,220 multiple cathodes are provided in a revolving drum such that the cathodes may be interchanged without breaking the vacuum seal of a vacuum chamber in which electric discharges occur. U.S. Pat. No. 4,713,170 discloses a water purifying system in which multiple cathodes are spaced around an anode. This multi-cathode configuration is used for decreasing the disturbance on the flow of water passing through the purifier. In U.S. Pat. No. 5,089,707, a multiple cathode assembly of electrically insulated cathodes are used for extending the life of an ion beam apparatus by alternating a cathode involved in the electric arc generation. In U.S. Pat. No. 5,225,625 multiple parallel cathodes, spaced from each other, are used in a plasma spray device for expanding the cross section of the plasma flow to prevent clogging of a plasma channel with powder particles. In general, prior art references disclosing multiple cathodes are not concerned with problems associated with generation of pulsed plasma.
Accordingly, there is presently a need for a cathode assembly and a method of operating of a device using the cathode assembly that would overcome limitations of the prior art for truly pulsed plasma generation.